Aquatic ape hypothesis

The aquatic ape hypothesis (AAH), also referred to as aquatic ape theory (AAT) and more recently the waterside model,[1] is the idea that the ancestors of modern humans were more aquatic. The hypothesis in its present form was proposed by the marine biologistAlister Hardy in 1960 who argued that a branch of apes was forced by competition from life in the trees to hunt for food such as shellfish on the sea shore and that this explained many characteristics such as man's upright posture. This proposal was noticed by Elaine Morgan, a scriptwriter, who objected to the male image of the "mighty hunter" being presented in popular anthropological works by Raymond Dart and others. While her 1972 book The Descent of Woman was very popular with the public, it attracted no attention from scientists, who saw no way of testing assertions about soft body parts and human habits in the distant past.

Morgan removed the feminist polemic in several later books, and her ideas were discussed at a 1987 conference devoted to the idea.[2] Her 1990 book Scars of Evolution received some favorable reviews, but the thesis was subject to scathing criticism from the anthropologist John Langdon in 1997 who characterized it as an "umbrella hypothesis" and argued that the hypothesis is not more parsimonious than simply rejecting the hypothesis.[3]

Though much of the relevant mainstream academic community ignored or derided the proposal, a few academics in the last 15 years have conducted research at least in part inspired by AAH.[4] Scientists supportive of AAH have published research that indicates that at some point in the last five million years humans became dependent on essential fatty acids and iodine, which are found in abundance in sea resources. Efficient function of the human brain requires these nutrients.[5][6][7]

The entirety of the "aquatic ape" proposal remains highly controversial, and is more popular with the lay public than with scientists.[8]

The German pathologistMax Westenhöfer (1871–1957) discussed in 1942 various human characteristics (hairlessness, subcutaneous fat, the regression of the olfactory organ, webbed fingers, direction of the body hair etc.) that could have derived from an aquatic past, quoting several other authors who had made similar speculations. As he did not believe human beings were apes, he believed this might have been during the Cretaceous, contrary to what is possible given the geologic and evolutionary biology evidence available at the time.[9] He stated: "The postulation of an aquatic mode of life during an early stage of human evolution is a tenable hypothesis, for which further inquiry may produce additional supporting evidence."[10] He later abandoned the concept.[11]

Independently of Westenhöfer's writings, the marine biologistAlister Hardy had since 1930 also hypothesized that humans may have had ancestors more aquatic than previously imagined, although his work, unlike Westenhöfer's, was rooted in the Darwinian consensus. On the advice of his colleagues, Hardy delayed presenting the hypothesis for approximately thirty years.[12][13] After he had become a respected academic and knighted for contributions to marine biology, Hardy finally voiced his thoughts in a speech to the British Sub-Aqua Club in Brighton on 5 March 1960. Several national newspapers reported sensational presentations of Hardy's ideas, which he countered by explaining them more fully in an article in New Scientist on 17 March 1960: "My thesis is that a branch of this primitive ape-stock was forced by competition from life in the trees to feed on the sea-shores and to hunt for food, shellfish, sea-urchins etc., in the shallow waters off the coast."[13]

The idea was generally ignored by the scientific community after the article was published. Some interest was received, notably from the geographer Carl Sauer whose views on the role of the seashore in human evolution[14] "stimulated tremendous progress in the study of coastal and aquatic adaptations" inside marine archaeology.[15] In 1967, the hypothesis was mentioned in The Naked Ape, a popular book by the zoologist Desmond Morris, who reduced Hardy's phrase "more aquatic ape-like ancestors" to the bare "aquatic ape", commenting that "despite its most appealing indirect evidence, the aquatic theory lacks solid support".[16]

While traditional descriptions of 'savage' existence identified three common sources of sustenance: gathering of fruit and nuts, fishing and hunting,[17] in the 1950s, the anthropologist Raymond Dart focused on hunting and gathering as the likely organizing concept of human society in prehistory,[18] and hunting was the focus of the screenwriterRobert Ardrey's 1961 best-seller African Genesis. Another screenwriter, Elaine Morgan, responded to this focus in her 1972 Descent of Woman, which parodied the conventional picture of "the Tarzanlike figure of the prehominid who came down from the trees, saw a grassland teeming with game, picked up a weapon and became a Mighty Hunter,"[19] and pictured a more peaceful scene of humans by the seashore. She took her lead from a section in Morris's 1967 book which referred to the possibility of an Aquatic Ape period in evolution, his name for the speculation by the biologist Alister Hardy in 1960. When it aroused no interest in the academic community, she dropped the feminist polemic and wrote a series of books–The Aquatic Ape (1982), The Scars of Evolution (1990), The Descent of the Child (1994) and The Aquatic Ape Hypothesis (1997)–which explored the issues in more detail.

My thesis is that a branch of this primitive ape-stock was forced by competition from life in the trees to feed on the sea-shores and to hunt for food, shell fish, sea-urchins etc., in the shallow waters off the coast. I suppose that they were forced into the water just as we have seen happen in so many other groups of terrestrial animals. I am imagining this happening in the warmer parts of the world, in the tropical seas where Man could stand being in the water for relatively long periods, that is, several hours at a stretch.[20]

Hardy argued a number of features of modern humans are characteristic of aquatic adaptations. He pointed to human's lack of body hair as being analogous to the same lack seen in certain other marine mammals, such as the whale and the hippopotamus,[21][22] and noted the layer of subcutaneous fat humans have that Hardy believed other apes lacked, although it has been shown that captive apes with ample access to food have levels of subcutaneous fat similar to humans.[23][24] Additional features cited by Hardy include the location of the trachea in the throat rather than the nasal cavity, the human propensity for front-facing copulation, tears and eccrine sweating, though these claimed pieces of evidence have alternative evolutionary adaptationist explanations that do not invoke an aquatic context.[25] Hardy additionally posited that bipedalism evolved first as an aid to wading before becoming the usual means of human locomotion,[13][26] and tool use evolved out of the use of rocks to crack open shellfish.[27][23] These last arguments were cited by later proponents of AAH as an inspiration for their research programs.

Morgan summed up her take on the hypothesis in 2011:

Waterside hypotheses of human evolution assert that selection from wading, swimming and diving and procurement of food from aquatic habitats have significantly affected the evolution of the lineage leading to Homo sapiens as distinct from that leading to Pan.[28]

Academics who have commented on the aquatic ape hypothesis include categorical opponents (generally members of the community of academic anthropology) who reject almost all of the claims related to the hypothesis. Other academics have argued that the rejection of Hardy and Morgan is partially unfair given that other explanations which suffer from similar problems are not so strongly opposed. A conference devoted to the subject was held at Valkenburg, Netherlands in 1987. Its 22 participants included academic proponents and opponents of the theory and several neutral observers headed by the anthropologist Vernon Reynolds of Oxford University. His summary at the end was:

Overall, it will be clear that I do not think it would be correct to designate our early hominid ancestors as 'aquatic'. But at the same time there does seem to be evidence that not only did they take to water from time to time but that the water (and by this I mean inland lakes and rivers) was a habitat that provided enough extra food to count as an agency for selection.[29]

The AAH has received little attention from mainstream anthropologists and paleoanthropologists. It is not accepted as empirically supported by the scholarly community,[30][31][32] and has been met with significant skepticism.[33]

In a 1997 critique, anthropologist John Langdon considered the AAH under the heading of an "umbrella hypothesis" and argued that the difficulty of ever disproving such a thing meant that although the idea has the appearance of being a parsimonious explanation, it actually was no more powerful an explanation than the null hypothesis that human evolution is not particularly guided by interaction with bodies of water. Langdon argued that however popular the idea was with the public, the "umbrella" nature of the idea means that it cannot serve as a proper scientific hypothesis. Langdon also objected to Morgan's blanket opposition to the "savannah hypothesis" which he took to be the "collective discipline of paleoanthropology". He observed that some anthropologists had regarded the idea as not worth the trouble of a rebuttal. In addition, the evidence cited by AAH proponents mostly concerned developments in soft tissue anatomy and physiology, whilst paleoanthropologists rarely speculated on evolutionary development of anatomy beyond the musculoskeletal system and brain size as revealed in fossils. After a brief description of the issues under 26 different headings, he produced a summary critique of these with mainly negative judgments. His main conclusion was that the AAH was unlikely ever to be disproved on the basis of comparative anatomy, and that the one body of data that could potentially disprove it was the fossil record.[3]

Anthropologist John D. Hawks wrote that it is fair to categorize the AAH as pseudoscience because of the social factors that inform it, particularly the personality-led nature of the hypothesis and the unscientific approach of its adherents.[34] Physical anthropologist Eugenie Scott has described the aquatic ape hypothesis as an instance of "crank anthropology" akin to other pseudoscientific ideas in anthropology such as alien-human interbreeding and Bigfoot.[35]

In The Accidental Species: Misunderstandings of Human Evolution (2013), the Nature editor Henry Gee remarked on how a seafood diet can aid in the development of the human brain. He nevertheless criticized the AAH because "it's always a problem identifying features [such as body fat and hairlessness] that humans have now and inferring that they must have had some adaptive value in the past." Also "it's notoriously hard to infer habits [such as swimming] from anatomical structures".[36]

Popular support for the AAH has become an embarrassment to some anthropologists who want to explore the effects of water on human evolution without engaging with the AAH, which they consider "emphasizes adaptations to deep water (or at least underwater) conditions". Foley and Lahr suggest that "to flirt with anything watery in paleoanthropology can be misinterpreted", but argue "there is little doubt that throughout our evolution we have made extensive use of terrestrial habitats adjacent to fresh water, since we are, like many other terrestrial mammals, a heavily water-dependent species." But they allege that "under pressure from the mainstream, AAH supporters tended to flee from the core arguments of Hardy and Morgan towards a more generalized emphasis on fishy things."[37]

Paleoanthropologists Alice Roberts and Mark Maslin published a critique of the AAH in the ‘i’ newspaper,[38] which they also published online, dismissing it as a distraction "from the emerging story of human evolution that is more interesting and complex", adding AAH has become "a theory of everything" that is simultaneously "too extravagant and too simple". They were responding to two BBC radio documentaries on “The Waterside Ape” made by David Attenborough[1] and aired in September 2016, in which consideration was given of a "move towards mainstream acceptance” for the AAH in the light of new research findings. Scientists interviewed about their research programmes included Kathlyn Stewart and Michael Crawford who had published papers in a special issue of the Journal of Human Evolution[39] on “The Role of Freshwater and Marine Resources in the Evolution of the Human Diet, Brain and Behavior”.

Positive reactions as part of broader critiques of paleoanthropology[edit]

Philosopher Daniel Dennett, in his discussion of evolutionary philosophy,[40] commented "During the last few years, when I have found myself in the company of distinguished biologists, evolutionary theorists, paleoanthropologists and other experts, I have often asked them to tell me, please, exactly why Elaine Morgan must be wrong about the aquatic theory. I haven’t yet had a reply worth mentioning, aside from those who admit, with a twinkle in their eyes, that they have also wondered the same thing." He challenged both Elaine Morgan and the scientific establishment in that "Both sides are indulging in adapt[at]ionist Just So stories". Along the same lines, historian Erika Lorraine Milam noted that independent of Morgan's work, certain standard explanations of human development in paleoanthropology have been roundly criticized for lacking evidence while being based on sexist assumptions.[41] Anatomy lecturer Bruce Charlton gave Morgan's book Scars of Evolution an enthusiastic review in the British Medical Journal in 1991, calling it "exceptionally well written" and "a good piece of science".[42]

In 1995, paleoanthropologist Philip Tobias declared that the savannah hypothesis was dead, because the open conditions did not exist when humanity's precursors stood upright and that therefore the conclusions of the Valkenberg conference were no longer valid. Tobias praised Morgan's book Scars of Evolution as a "remarkable book" though he said that he did not agree with all of it.[43][44] Tobias and his student further criticised the idea by arguing that the coming out of the forest of man's precursors had been an unexamined assumption of evolution since the days of Lamarck, and followed by Darwin, Wallace and Haeckel, well before Raymond Dart used it.[45]

Alister Hardy was astonished and mortified in 1960 when the national Sunday papers carried banner headlines "Oxford professor says man a sea ape", causing problems with his Oxford colleagues.[46] As he later said to his ex-pupil Desmond Morris, "Of course I then had to write an article to refute this saying no this is just a guess, a rough hypothesis, this isn't a proven fact. And of course we're not related to dolphins."[1]

Elaine Morgan's 1972 book Descent of Woman became an international best-seller, a Book of the Month selection in the United States and was translated into ten languages.[47] Part of this was related to the growing women's liberation movement as Morgan's work was inspired by a feminist critique of then-standard anthropological ideas. She assumed that the total lack of response to her book from the academic community was due to the fact that she was an outsider. "The response I had not foreseen was total silence. But in respect of the aquatic theme that is what I got from them - and with few exceptions still get. That kind of silence is a virtually unbeatable strategy".[48] Morgan removed the polemics and rewrote the scientific part publishing it as The Aquatic Ape ten years later, but it did not gain more acceptance from scientists.[49]

Although the general reaction to Hardy and Morgan's proposals was silence by the relevant academic community, there have been over the last decades some academics who were inspired by AAH proposals, even to the point of pursuing particular lines of research on its basis. Some of the academics and professional scientists who have supported the AAH include Michael Crawford, Professor and Director of the Institute of Brain Chemistry and Human Nutrition at Imperial College London, his former postdoctoral researcher, Stephen Cunnane, now professor of medicine at Université de Sherbrooke, Erika Schagatay, professor of Environmental Physiology at Mid Sweden University, and Kathlyn M. Stewart, Section Head of Palaeobiology at the Canadian Museum of Nature.

After reviewing 30 different explanations of bipedalism, evolutionary biologist Carsten Niemitz proposed a "shore dweller" hypothesis of wading, though distancing himself from the AAH.[50][51][52] He records 10 monkey and ape species, including chimpanzees and gorillas, who have been observed wading on two feet.[53] Niemitz's hypothesis places the wading bipedalism of hominids as occurring in the late Miocene.[50] Other researchers, including AAH proponent Algis Kuliukas, performed experiments to measure the comparative energy used when lacking orthograde posture with using fully upright posture. Although it is harder to walk upright with bent knees on land, this difference gradually diminishes as the depth of water increases[54] and is still practical in thigh-high water.[55]

In a critique of the AAH, Henry Gee questioned any link between bipedalism and diet. Gee writes that early humans have been bipedal for 5 million years, but our ancestor's "fondness for seafood" emerged a mere 200,000 years ago.[56]

In their 1989 book The Driving Force: Food, Evolution and The Future, Michael Crawford together with co-author David Marsh wrote about the connections they saw between nutrition and human evolution, and in particular claimed that Omega-3 fatty acids were vital for the development of the brain.[57] "A branch of the line of primitive ancestral apes was forced by competition to leave the trees and feed on the seashore. Searching for oysters, mussels, crabs, crayfish and so on they would have spent much of their time in the water and an upright position would have come naturally."[58] Crawford and Marsh argued that the norm of brain size seen in small and aquatic mammals is similar to humans and they further opine that other primates and carnivores had lost relative brain capacity (though the relationship between brain size and intelligence is not isomorphic).[59] Reviewing their book for the New Scientist, biologists Caroline Pond and Dick Colby were highly critical, saying that the work provided "no significant new information that would be of interest to biologists" and that its style was "speculative, theoretical and in many places so imprecise as to be misleading."[60]

According to Crawford, "the evolution of a large brain on the savannahs of Africa was impossible. The only way it could have happened was with the resources of the marine food-web which initiated the growth and development of the brain in the first place".[1] A group of evolutionary nutritionists and paleontologists led by Cunnane, Stewart, and Crawford published works arguing a correlation between aquatic diet and human brain evolution in their "shore-based diet scenario",[5][61][62] acknowledging the Hardy/Morgan's thesis as a foundation work of their model.[63] They describe problems in landlocked communities as being supportive of AAH, with reports of cretinism in the Alps since the 18th century and widespread goitres in parts of Africa, which is only prevented by the addition of iodine to diets.[64][65] According to Cunnane, "Without the availability of iodized table salt, non-coastal (inland) habitats do not provide foods rich enough to meet human iodine requirements."[66] Evidence supports aquatic food consumption in Homo as early as the Pliocene[67] but its linkage to brain evolution remains controversial.[68][69]

What has been lacking has been the paleontological evidence that early humans consumed fish in significant amounts earlier than tens to hundreds of thousands of years ago.[70] Part of the problem has been the avoidance of taphonomic bias by researchers: most hominin fossils occur in marginal lacustrine environments and the presence of fish remains is therefore no proof of fish consumption. Fish bones often decompose so that special tests are required to detect them.[71] The archaeological record of human fishing and coastal settlement is fundamentally flawed due to postglacialsea level rise.[72]

Aside from working as a professor, Schagatay is also an experienced scuba and freediver whose research centers around human diving abilities. She suggests that such abilities are consistent with selective pressure for underwater foraging during human evolution, and discussed other anatomical traits speculated as diving adaptations by Hardy/Morgan.[73] John Langdon suggested that such traits could be enabled by a human developmental plasticity.[74]